The following acronyms and definitions are used in elaborating the principles disclosed herein.
Acronyms and Definitions
1. Acronyms
    ACG Automatic Code Gapping    AMA Automatic Message Accounting    ANSI American National Standard Institute    CdPN Called Party Number    CPE Customer Premises Equipment    FCI Forward Call Indicator    GAP Generic Address Parameter    GTAI Global Title Address Information    GTT Global Title Translation    IAM Initial Address Message    ISDN Integrated Services Digital Network    ISUP ISDN User Part    LARG LIDB Access Routing Guide    LATA Local Access Transport Area    LERG Local Exchange Routing Guide    LIDB Line Information Database    LNP Local Number Portability    LRN Location Routing Number    LSMS Local Service Management System    LU Living Unit    NP Number Portability    NPA Numbering Plan Area    NPAC Number Portability Administration Center    NPDB Number Portability Database    NXX Central Office Code    SCCP Services Connection Control Part    SMS Service Management System    SS7 Signaling System 7    STP Signal Transfer Point    TCAP Transaction Capabilities Application Part    TN Telephone Number
2 DefinitionsBlock HolderThe switch designated by the service provider toSwitchwhich a thousands block (NXX-X)1 has beenassigned for use and allocation to end users.Code Holder SwitchThe switch designated by the service provider towhich the NXX is assigned in the LERG.Default RoutingThe ability of the switch to continue a call based ondialed number when NPDB cannot be accessed dueto abnormal circumstances or when NPDB responsecontains a protocol error.Donor SwitchThe switch from which a TN was originally ported.In a number pooling environment, the donor switchis the code holder switch.End UserBusiness or residential subscriber.Home LRNAn LRN is a Home LRN for a switch if the LRNmay be used as a routing number to complete callsto TNs that have been pooled-in or ported-in to theswitch (i.e., this LRN is associated with the TNs inthe NPDB). To qualify as a home LRN for a givenswitch, the NPA-NXX of that LRN must beassigned to that switch in the LERG.Intermediate SwitchA tandem switch.Local AccessA defined geographic area where equal accessTransportswitches or access tandem switches can provideArea (LATA)carrier access to the local switch.Line (TN) ServedAny TN that is connected to the switch or subtendsby Switchthe switch. The TN may be a physical subscriberconnection or a virtual TN.Location RoutingA 10-digit number in the format NPA-NXX-XXXX.NumberThe first 6 digits of the LRN identify a switch.NP-Capable SwitchA switch that has the NP capabilities described inTechnical Requirements for Number Portability -Switching Systems.NP QueryA request for call routing information sent from theswitch to the NPDB when a call encounters an NPtrigger.NP-ReservedA switch marking on a TN or range of TNs.MarkingOperational UserThe service provider's maintenance personnel.Originating SwitchThe switch serving the calling party.Pooled NumberA number within a pooled and allocated block ofTNs that is working, assignable, or aging in theblock holder switch. A pooled number will have aLocal Number Portability (LNP) Type2 of POOL inthe NPAC SMS.PoolingEntity responsible for the administration of theAdministratorNXX-X blocks within an NPA.Portable NPA-NXXAn NPA-NXX designated as “open” for portability.It may be the case that no numbers have actuallyported.Ported NumberOutside of a pooled block, a TN is ported if it isworking, assignable, or aging in a switch other thanthe code holder switch. Within a pooled block, a TNis ported if it is working, assignable, or aging in aswitch other than the block holder switch. A portednumber will have an LNP Type2 of LSPP or LISP inthe NPAC SMS.Ported Out MarkingA switch marking on a TN or range of TNs.Rate CenterA geographic area used to distinguish rateboundaries. In this document “rate center” denotesthe smallest geographic area used to distinguish rateboundaries. In other contexts, rate centers maycontain even smaller geographic areas used forrating (e.g., rate districts, wire centers, rate areas).Recipient SwitchThe switch to which a TN is ported.SnapbackThe event whereby the responsibility for a portedTN is returned to the block holder (if the TN is in apooled block) or the code holder (if the TN is not ina pooled block) after disconnect and aging.Unallocated NumberA TN is unallocated on a switch if it is not assignedto a customer on the switch.1The term NXX-X refers to the NPA-NXX-XXXX of the North American Numbering Plan.2The value of LNP Type is assigned in the NPAC SMS National Number Pooling Requirements
Before Congress enacted the Telecommunications Act of 1996, competition between telecommunication carriers was minimal. Central Office codes commonly known as a NXX (NPA-NXX-XXXX) or a block of 10,000 numbers, were in plentiful supply and readily available to existing Local Exchange Carriers (LECs). Little thought was ever given to telephone number conservation. If a LEC inadvertently asked for more numbers than it needed, it would hold the numbers until a future date, when the numbers might be used. It was not uncommon to see CO Codes assigned to a LEC, and remain unused for years. During that period, it was rare to see a CO Code taken out of service or returned, so that the CO Code could be reassigned. In the rare case where a CO Code was taken out of service, or returned, it was a relatively simple matter to give the CO Code back and then reassign the code. If there were a few working customers in the CO Code, they were simply given a telephone number change to remove the customer from the CO Code being returned.
With the implementation of the Telecommunications Act of 1996, however, competition became a reality as numerous new Competitive Local Exchange Carriers (CLECs) entered the market. CLECs entering into business for the first time required telephone numbers to serve customers in the markets they were entering. Additionally, with the introduction of Local Number Portability (LNP), CLECs needed a way to be able to port a customer away from a competitor. In addition to CLECs new to the market, Cellular service has seen dramatic growth. Further, new products came into being, which required additional telephone numbers. All of this resulted in opening new CO Codes at an increasing rate.
As new CO Codes are opened, regulators and industry looked for ways to minimize the opening of new CO Codes and to maximize the use of existing numbers in CO Codes which have already been opened. One of the methods, which has been, and continues to be used successfully, is Number Pooling. Other methods have included Rate Center Consolidation (RCC), sequential numbering, and utilization percentages which companies are required to reach before new resources will be assigned (blocks or CO Codes).